In-situ pellicle monitor

ABSTRACT

A mask structure and method of quantitatively measuring pellicle degradation in production photomasks by measuring overlay in test structures on the mask. A structure is located in a high transmission region close to a transition region between a low transmission and a high transmission region of the mask such that pellicle degradation impacts the printing of the object. A second structure is located in low transmission region such that the printing of the second structure overlaps the first and provides a measure of pellicle degradation.

BACKGROUND OF THE INVENTION

This invention relates to monitoring a pellicle to determine when itwears out. More particularly it relates to methods and structures tomonitor pellicles subject to exposure wavelengths that can causepellicle thinning.

Typically people look at the pellicle in monochromatic light and lookfor signs that the mask pattern is “printing” onto the pellicle surface.The technique is not quantitative and not very sensitive. It also issensitive to the wrong thing, namely thin-film interference within thepellicle, not externally measured change in optical path length. Thinfilm interference is sensitive to changes in nt (where n is therefractive index and t is the thickness), while external optical pathlength is sensitive to changes in (n−1)t.

An example of another system that is intended to determine pellicle lifeis provided by Japanese Application Number JP19870111206 19870506entitled “Exposure Device” by Mitsubishi Electric Corporation. In thatpublished application a light flux from an optical system is focused ona photo detector for measuring light intensity at wafer level. The lightintensity is measured with and without a pellicle to determine thetransmissivity of the pellicle. When the transmissivity of the pelliclefalls below a certain set value the pellicle is considered worn out.Again this is another way of determining wear out by transmission loss.

Pellicles can be degraded by the exposure light during use. For 365 and248 nm, this has not been a major problem, because the pelliclematerials in use are very resistant to damage at those wavelengths. Atincreasingly short wavelengths, such as 193 and 157 nm, pellicles aremuch more easily damaged. Damage can show up as a change intransmission, thickness, index of refraction, or a combination of allthree. Changes in pellicle transmission lead to dose changes duringwafer exposure. This will cause image size changes which can readily bedetected.

BRIEF SUMMARY OF THE INVENTION

However, non-uniform changes in optical path length of the pellicle (afunction of thickness and index of refraction) can cause image positiondisplacements which are not easily detected. It has proven extremelydifficult to quantitatively measure the amount of optical path lengthchange induced by photo-induced pellicle damage. This means that thereis a risk of using a pellicle that has begun to induce opticaldistortions to the mask beneath it, or conversely to discard a pellicleout of excessive caution, before its life is over. This inventionteaches a mask structure and method of quantitatively measuring pellicledegradation in production photomasks, by measuring overlay in teststructures on the mask. A set of test structures that can be measuredare placed in a transition region where the relative movement betweenthem provides an indication of degradation. Once a predetermineddisplacement has taken place that impacts the quality or yield of thelithography process, the pellicle is considered worn out. Referencestructures and additional monitor structures can be added to the mask tocorrect for displacement due to stepper error. Even more particularly amask which comprises a first and second test structure; one structurelocated in a high transmission region close to a transition boundarybetween high and low transmission regions of the mask such that pellicledegradation impacts the printing of the object; the second structurelocated in the low transmission region of the mask close to thetransition boundary such placement such that the structures will overlayeach other. A second set of structures is used for control purposes tosee if the difference in overlay is due to other factors The method thencompares these structures to determine the image displacement due todegradation that is taking place.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates in diagram form non-uniform pellicle thinning.

FIG. 2 illustrates in diagram form how image displacement occurs at aboundary between high transmission and low transmission area due topellicle degradation.

FIG. 3 illustrates in diagram form how structures could be placedadjacent a transition area to measure this pellicle degradation.

FIG. 4 illustrates in diagram form how the wear out monitor will printon the wafer as a clear, square opening in a square pad of resist.

FIG. 5 illustrates in diagram form where the wear out monitor should beplaced relative to the edge of the exposure field.

FIG. 6 illustrates in diagram form the layout of the opaque monitorstructure on a mask near the frame.

FIG. 7 illustrates in diagram form the layout of a clear monitorstructure in the frame.

FIG. 8 illustrates in diagram form the layout of the monitor structuresas they would appear on a mask near the frame.

FIG. 9 illustrates in diagram form the layout of two sets of symmetricalmonitor structures.

FIG. 10 illustrates in diagram form the layout of monitor structures andreference structures.

DETAILED DESCRIPTION OF THE INVENTION

The pellicle wear out monitor of this invention is designed to detectnon-uniform pellicle thinning, which can lead to intra-field imageplacement errors. Pellicle thinning is caused by damage to the pelliclematerial induced by exposure to the stepper illumination source. This isnot expected to occur, and has not been seen in i-line or 248 nmsteppers, as long as the correct pellicle material is used at thosewavelengths. Lab measurements of pellicle materials indicate a strongpotential for pellicle thinning at the 193 nm exposure wavelength. It isdifficult to make a quantitative estimate of the expected lifetime ofpellicles used at 193 nm, partly because of uncertainty in theexperimental tests of the material, and partly because of the practicaldifficulty of tracking the exact exposure that a pellicle has seenthroughout its lifetime in the manufacturing line.

One can theoretically understand the relationship between pelliclethinning and shifts in image placement. The main uncertainties arerelated to the amount of exposure seen by the pellicle and the amount ofthinning caused by a given level of exposure. Testing indicates thatsubstantial thinning may begin to occur at integrated doses of between 2kJ/cm² and 5 kJ/cm² of 193 nm radiation. In comparison, pellicles usedat 248 nm exposure wavelength have quoted lifetimes of 500 kJ/cm².

When the pellicle thins, there is typically a region of imagedisplacement at the boundary between high transmission and lowtransmission regions of the mask, as between a DRAM array and thesurrounding support areas, or between the product and kerf areas of themask. The image displacement is caused by uneven thinning of thepellicle between the highly illuminated areas and the dark areas of themask. The pellicle wear out monitor of this invention is intended to beused on bright field photomasks that will be exposed at any wavelengthlikely to cause damage to the pellicle. It is placed at a sharp boundarybetween a large bright area and a large dark area for maximumsensitivity to uneven pellicle thinning. Ideally, it should be placed atthe boundary between the bright field exposure area and the opaquechrome frame. This region should have the maximum sensitivity topellicle thinning of any part of the exposure field. The pellicle wearout monitor described herein is designed to magnify the measured imageplacement error by 2 times, which will give an additional safety marginwhen determining when the pellicle is worn out and must be replaced.

The most severe image placement problem occurs at the boundary between alarge clear region of a mask and a large opaque region. As seen in FIG.1 light passing through the clear region 1 does maximum damage to thepellicle lying over that region, while the pellicle lying above theopaque region 5 is not damaged. Between the two regions lies a section 3of pellicle 6 that is exposed to a gradient of light intensity, smoothlyincreasing between the dark area and the fully illuminated area. As seenin FIG. 2 if the pellicle is thinned proportionally to the amount oflight exposure it has received, it will develop a wedge-shapedcross-section 3 in the partially illuminated area. This wedge-shapedregion will act as a very weak prism, deflecting by a tiny amount thelight that passes through it. Because of the relatively large distance hbetween the pellicle and the reticle (3–5 mm), the tiny displacementangle can lead to several tens or hundreds of nanometers of image shiftdx. Only the images that lie under the tapered part of the pellicle aredisplaced by a pellicle damaged in this way. Features in a uniformlythinned region 2 of the pellicle 6 in FIG. 1 or an undamaged region 4 inFIG. 1 are not displaced at all. This produces an effective opticaldistortion of the mask surface, with images at the edges of large brightregions being displaced inward toward the bright region, and regions atedges of large dark regions being displaced outward. The transversedimensions of the tapered region of the pellicle are determined by thepellicle standoff height and the numerical aperture of the stepperilluminator. For a typical pellicle height of 4 mm and illuminator NA of0.075, the width of the tapered region will be about 600 um. Maskobjects centered beneath this tapered region will be displaced, with thedisplacement gradually disappearing for objects further away from thetapered region.

The masks most seriously affected by pellicle degradation arebright-field masks. Dark-field masks (e.g. contact hole arrays) provideso little optical exposure to the pellicle that there is generally nodegradation. The transition from the exposure field to the opaque frameof a dark-field mask provides a worst-case test area to evaluatepellicle degradation. For the purposes of the specification hightransmission regions and clear are often used synonymously. Likewise lowtransmission regions and opaque regions are often used synonymously. Thepoint is that regions do not have to be entirely opaque or entirelyclear in order to apply the teachings of this invention. FIG. 3illustrates how structures could be placed adjacent a transition area tomeasure pellicle degradation.

On each side of the mask 10, two opaque squares 11 and 12 are placed,one (B) very close to the opaque frame (e.g. within 50 um of the frame)and the other (A) far enough from the edge that it is completely outfrom under the tapered area of the pellicle (typically <600 um from theframe). Two corresponding clear squares (C and D), slightly smaller thanthe opaque squares just described, are placed in the opaque frame 13 insuch a way that they will exactly overlay the opaque squares from aneighboring exposure field when printed on a wafer. When printed on thewafer, these overlaid structures will create a box-in-box featureideally suited for overlay measurements. Such interlocking structuresare commonly used to discover printing errors such as die rotation,magnification errors, or wafer scale errors. In fact, the set of squaresfarthest from the boundary (A and C) between the opaque frame and theclear exposure field will show exactly these sorts of printing errors.However, the set of squares closest (D and B) to the exposure fieldboundary must show exactly the same placement errors as the more remotesquares if the pellicle is perfect. Degradation of the pellicle willexhibit a different apparent field magnification (or wafer scale) errorfor the inner set of squares vs the outer set, because the one set ofsquares sits under the area of the pellicle most susceptible to taperformation, while the other set does not.

The use of a dual set of structures in FIG. 3 when printed measures morethan pellicle degradation. Rotation, skew, and magnification can becalculated from the interlocking structures A and C. The interlockingstructures B and D give the same information, except that the differencebetween magnification from A and C and magnification from B and D givesa measure of the distortion induced by pellicle aging. All of the othercomponents of overlay should be identical, and give redundancy for errorchecking and measurement accuracy estimation. Because B is displacedinward and D is also displaced inward by a damaged pellicle, the overlayof B and D will double the sensitivity of the measurement.

If structures C and D are obscured by the framing blades of the stepper,the mask could be exposed on a monitor wafer with the framing bladeswithdrawn to expose C and D. This test could be done at establishedintervals just to monitor the health of the pellicle.

Details of an exemplary embodiment are the following:

1. In the preferred embodiment of this invention the wear out monitorwill print on the wafer as a clear, square opening in a square pad ofresist. These are shown as 20 and 21 in FIG. 4. The designer can selectthe dimensions of the resist square and the clear square based on therequirements of the metrology equipment used to measure the overlay. Themonitor structure 16 will be measured for overlay of the clear squarewithin the resist square. Typical dimensions could be 10×10 Fm for theresist square 21 and 5×5 Fm for the clear opening 20.

2. Referring to FIG. 5 the wear out monitor 16 should be placed as closeto edge 22 of the exposure field as possible, without interfering withthe ability to measure the monitor for overlay.

3. Referring to FIGS. 6, and 8 the actual layout of the monitor consistsof two parts, an opaque square 24 near the chrome frame 25 of a brightfield mask and a clear square 26 in frame 25 on the opposite side of themask as shown in FIGS. 7 and 8. The separation between the centers ofthe two squares 24 and 26 must be exactly equal to the steppingdistance, so the clear and opaque squares will interlock when thepattern is printed.

The particulars of placement are the following:

1. The two squares which make up the wear out monitor must be close tothe chrome frame in order to allow them to interlock when the pattern isstepped onto the wafer. Also, the sensitivity of the measurement will bemaximum at the boundary between a large clear area and the opaque frame.The worst image shifts will occur within a distance of:

$x = \frac{{NA} \cdot \sigma \cdot h}{{Mag}^{2}}$

on each side of the frame boundary, but residual image shifts will beseen up to a distance of:

$x_{r} = \frac{{NA} \cdot \left( {\sigma + 1} \right) \cdot h}{{Mag}^{2}}$

In these equations, NA and F are the numerical aperture and partialcoherence factor used to expose the mask, Mag is the steppermagnification, and h is the pellicle standoff height. In the case ofannular illumination, F should be taken as the outer F of the annularillumination.

Example: A 4× stepper with NA=0.72, r=0.60, and pellicle height 4 mmwill have a region of maximum image shift within 108 Fm on each side ofthe frame edge, and residual image shifts will be seen out to a distanceof 288 Fm from the edge of the frame. The equations give x and x_(r) inwafer scale dimensions.

2. Both squares should be adjacent to relatively clear areas extendingat least 2× into the exposure field, where x is defined in the equationabove. “Relatively clear” means that the average transmission over theclear region should be as high as possible, preferably >90%. Maskpatterns within this zone will not degrade the sensitivity of themeasurement as long as the average intensity is high. If there is noregion meeting this specification, the squares should be placed adjacentto areas with the highest average transmission within a radius of 2×from the location of the squares. Both the clear and the opaque squaresare affected by the adjacent clear areas, so the location should bechosen to maximize the transmission adjacent to both squares. It ispreferred that the squares interlock, so they should be directly acrossfrom each other. Corner locations are not ideal, because half of thelight in the clear area is obstructed by the adjacent side of the frame.

3. If the wear out monitor is placed in an ideal, high contrast areanear the frame, it will indicate degradation in the pellicle long beforeit can affect product yield. There are certain design scenarios in whichthe monitor will not work. The worst case would be mask design with verylow transmission near the frame, but a large, highly transparent regionfar away from the frame. In this case, it is possible to look forpellicle degradation by monitoring overlay to a previous level, placingthe overlay measurement structures near the boundary between thetransparent region and adjacent opaque regions. In this case the wearout monitor structure would be embedded on masks for two levels. Onemask level would either have no pellicle or be exposed using a pelliclewhere no or little degradation has occurred, the other being in thetransition region where non-uniform degradation takes place.

4. The accuracy of the measurement could potentially be affected bymeasurement tool image shift (TIS) or other effects. If this is aconcern, a second set of wear out monitor structures, 34 and 36 in FIG.9, could be added to the design. This provides us with measurement sitesreflected symmetrically through the center line of the mask. Themonitors could be aligned vertically instead of horizontally, ifdesired, or a total of four monitors could be used, one on each side ofthe mask.

Details concerning measurement of the monitor are provide in thefollowing:

1. When the pellicle begins to degrade from exposure to the stepperillumination, the images of the squares will be slightly displacedtoward the bright areas of the mask. Both the clear square and theopaque square in the structure will be displaced toward the center oftheir exposure fields, but because the two parts of the structure areprinted in two adjacent fields, the relative displacements will be inopposite directions. When the centration of the clear square within theopaque square is measured with an overlay measurement tool, themeasurement will indicate a placement error twice as large as the worstplacement error actually present within the exposure field. Tolerancescan be defined to determine when the pellicle must be replaced.

2. Stepping and magnification errors in the stepper will induce errorsin reading the Pellicle wear out monitor. Placement errors from thesesources will affect the Monitor just as placement errors from a degradedpellicle do. If control of stepping scale and magnification is accurateenough, the errors measured by the wear out monitor can all beattributed to the pellicle. If the stepper errors are not sufficientlysmall to be ignored, then a second set of reference monitor structures44 and 46 can be added to the mask as shown in FIG. 10. If theseadditional reference monitors are placed in uniformly illuminated areas(regardless of the magnitude of the light transmission), then they willnot suffer image shifts when the pellicle thins, and can be used toremove any stepping and magnification errors. The net value of imageshifts caused by pellicle thinning would then be the difference betweenthe reference monitor and the pellicle wear out monitor measurements.

3. In a bright field pattern with very low pattern density, the twoparts of the reference monitor can be placed at least a distance x_(r)away from the edge of the chrome frame. The wear out monitor still needsto be as close to the edge of the frame as possible.

4. It is possible to measure the wear out monitor on product wafers, butit is preferred to make a special exposure on a monitor wafer. This willgive a planar resist surface for the exposures and remove the confusionof measuring over previous levels of patterning on the wafer. If areference monitor is used in addition to the wear out monitor, thestepper framing blades may need to be retracted to expose the clearsquare component of the reference monitor.

Example: For a level using a 40 mJ/cm² resist, with 75 exposure fieldsper wafer, using a 4× stepper whose lens has a 60% transmission, 500J/cm²F would be reached after 1600 wafer exposures.

While the embodiments provided in this application use squares asstructures and place them relative to the frame of a mask, thisinvention is by no means limited to the those structures and relativeplacements. Any structure that could be used to measure displacementcould be applied to this invention. Other simple examples would be a barwithin a bar or a frame within a frame. Nor do they have to be of thesame form, for example, a circle within a square, so long as layout andprintability issues are taken into account. Likewise, the structurescould be placed in other areas of a mask, where transitions from brightfield to dark fields take place and that allow one to comparedisplacement in these transition areas to areas where no displacementshould take place.

Therefore, while the invention has been described with reference to theembodiments provided herein, it is not confined to the details set forthherein. The specification and examples are only exemplary, so theinvention should be construed only with regards to the claim appendedhereto.

1. A method of determining pellicle degradation, comprising the stepsof: printing on a wafer with a stepper a mask image of monitorstructures which are located in a transition between clear and opaqueregions of the mask one stepping distance apart; measuring thedisplacement between the printed monitor structures; determiningpellicle degradation by the degree which the structures are displacedfrom each other.
 2. The method of claim 1 wherein one of the structuresprints as resist and the other as clear.
 3. The method of claim 1wherein the step of printing comprises two steps: printing a firststructure located in a transition region of the mask; and printing asecond structure at an adjacent level located in the same positionrelative to the first structure so that it overlays the first.
 4. Themethod of claim 1, wherein the stepper has its blades retracted duringthe wafer exposure.
 5. The method of claim 1, also comprising the stepof comparing the degree with which the structures are displaced with ameasured displacement measured using the pellicle when it was new.